1. Field of the Invention
The present invention generally relates to an optical disc manufacturing device for manufacturing disc substrates used for optical discs, a method of manufacturing optical discs by mutually adhering a first disc substrate and a second disc substrate, and to an optical disc.
2. Description of the Related Art
An optical disc such as DVD, DVD−R or DVD+R or the like is typically formed by mutual adhesion of a first disc substrate and a second disc substrate that are manufactured by injection molding. In the injection molding, an injection molding device for securing a single disc substrate is generally employed. However, recently, in order to improve the productivity and space efficiency for the device, an injection molding device for securing double disc substrates in which disc substrates are molded simultaneously to be secured in two molds is sought to be put in practical use (e.g., Japanese Patent Laid-open No. JP-A-8-224763).
In the injection molding device for securing double disc substrates, it is general to provide two fixed molds parallel to each other on a fixed platen with the two fixed molds spaced apart and provide two movable molds parallel to each other on a movable platen with the two movable molds spaced apart. However, in order to have much more injection molding devices installed in limited space, the width of the injection molding device should be shorter. Accordingly, it is necessary that the distance between the two fixed molds and the distance between the two movable molds, more specifically a distance between two molds be zero or near zero.
However, a heat radiating efficiency in a boundary part of the molds causes the temperature at the same part during the injecting process and the hardening process to be higher than those of the other parts, when the distance is zero or near zero.
The fixed molds and the movable molds constituting the mold is appropriately provided with temperature controlling flow passages for a heating medium to uniform the temperature throughout the mold in the injecting process and the hardening process. However, it is difficult to completely remove temperature differences even though concentrated studies on different shapes of the temperature controlling flow passages are made. As a result, the temperature difference is still transferred and generated in a plastic in cavities, thereby causing unevenness of thickness in the disc substrates molded by each of the molds.